Abnormalities in serotonin neurotransmission are implicated in numerous disorders from pediatric developmental disorders including the sudden infant death syndrome (SIDS), autism, Rett syndrome, and Prader-Willi syndrome1-6, to adult psychiatric disorders involving mind and mood. The pediatric disorders, especially, exhibit life-threatening respiratory instability that can lead to death. This clinical link between serotonin an respiratory abnormalities has motivated numerous investigations into the role of serotonergic neurons (5-HT neurons), particularly those in the lower brainstem, in regulating respiratory drive. Our lab has published data using an exciting set of new mouse genetic tools developed by us demonstrating that brain 5-HT neurons are capable of regulating the respiratory network, and thus lung ventilation, in response to elevated blood carbon dioxide (CO2) levels, and that at least a subset are also likely to act as central respiratory chemoreceptors directly sensing changes in pH/CO2 and transducing these changes into cellular activity output capable of affecting respiratory function to achieve homeostasis. Conditions plausibly leading to elevated CO2 levels (e.g. face-down infant rebreathing exhaled stale gases with high CO2 levels accumulated in a pocket formed by bedding materials;) are considered risk factors for sudden death in these disorders. Thus, understanding the molecular and developmental underpinnings of the specific 5-HT neurons involved in respiratory drive and CO2 chemosensitivity is a critical step, not only important to our understanding of homeostatic neural networks, but also towards (1) understanding brainstem abnormalities that pose risk for life-threatening events, (2) developing tests for assessing risk, and (3) ultimately for establishing interventions that might help prevent sudden death in infants and children. On this basis, we now hypothesize that within the 5-HT system is a specialized subset of 5-HT neurons uniquely equipped molecularly to sense and transduce changes in CO2/pH to affect respiratory drive, and that this functionality resides selectively in the 5-HT neuron lineage defined by developmental expression of the transcription factor Krox20 and originating from hindbrain rhombomere 5 (r5). We propose three aims directed at characterizing, through in vitro electrophysiology (Aim 1) and in vivo neuronal silencing (Aim 2), the functional contributions of specific 5- HT neuron subtypes to respiratory homeostasis, and to identify molecular factors (e.g. channels, receptors) associated with CO2 chemosensitivity (Aim 3) that may potentially underlie this vital function.